This invention relates to switches for producing a signal in response to increase and/or decrease in flow rate of a flow of liquid beyond a predetermined value, and to methods of using such switches. In a particular sense, it is directed to switches capable of detecting and signalling minor changes of flow in a gravity drain even at very low mass flow rates.
For various purposes, it is desirable to sense small changes in liquid flow rates under conditions of low mass flow. An example is in the lubricating systems employed in sheet metal rolling lines. In such a system, a so-called relubricating machine supplies oil to an open trough and several spray nozzles pick up oil from the trough for application to a surface to be lubricated. Excess oil flows out of the trough to a gravity drain and is recirculated. The location of the trough, in the apparatus with which it is associated, precludes convenient direct measurement or observation of the oil level in the trough. However, as long as a sufficient flow of oil leaves the trough through the gravity drain, it is known that adequate oil is present in the trough. Accordingly, it would be desirable to provide a switch, responsive to changes in oil flow rate in the gravity drain, for providing a signal representative of decrease of flow rate below a predetermined value, i.e. to detect insufficient lubricant runoff which indicates insufficient lubricant in the pumping section of the apparatus.
The mass flow rate of oil in the gravity drain, and the magnitude of the changes to be detected, are too small to be sensed by conventional switches such as vane-type flow switches. In particular, known commercially available flow switches do not operate without a flow having significant kinetic energy, i.e. kinetic energy in excess of that characteristic of the drain flows desired to be detected in the aforementioned lubricating operations.